Many disease processes such as solid tumors and liver fibrosis manifest themselves as changes in tissue mechanical properties. Manual palpation of tissues is an important clinical tool for the detection of disease within the body. Unfortunately, palpation is mostly limited to superficial organs and to the sensitivity of tactile perception.
Elastography is an imaging technique that uses tissue mechanical properties as the source of contrast. Advantages of elastography include the capability to quantify the elastic properties associated with disease and to provide a measure of the spatial distribution of elastic properties within organs to localize regions of altered stiffness. Many elastographic imaging techniques rely on measuring the displacements of tissue via the application of an external force on the tissue. The stiffness distribution is calculated based on the principle that the displacements exhibited by tissues in the presence of a force are determined by the underlying elastic properties. Ultrasound and magnetic resonance imaging (MRI) are both capable of resolving tissue displacements for elastography, and are both being developed in parallel for tissue stiffness imaging. Ultrasound techniques are typically based on tracking the displacement of scatterers along the direction of the ultrasound beam upon manual compression of tissue with a transducer. The technique is limited to measuring static elastic properties and is dependent on the operator.
Magnetic resonance elastography (MRE) is a promising technique that involves application of vibration using a mechanical actuator and measuring the resulting tissue motion using phase-contrast MRI sequences capable of resolving particle displacement in all three dimensions. Dynamic mechanical properties are measured by generating shear waves in the frequency range of 50-1000 Hz. These wave images are used to calculate elastograms, or stiffness maps, in tissue, which can be used as diagnostic tools.
The source of mechanical vibration in MRE is typically applied at the external surface of the body, causing mechanical shear waves to propagate into the body. Unfortunately, shear waves are highly attenuated in the body and are typically limited to less than 10 cm of penetration at frequencies between 50-60 Hz. In addition, the low frequencies used to achieve sufficient penetration result in longer shear wavelengths in tissue that result in lower spatial resolution elastograms.
It is therefore desirable to adapt MRE for the imaging of deep-seated organs at diagnostically viable spatial resolutions. Recently, a system for performing MRE was disclosed by Smith in U.S. Pat. No. 6,862,468, in which an ultrasonic transducer is mounted at the distal end of a catheter. The catheter is inserted into the subject and shear waves are generated locally at the distal end of the catheter by the ultrasonic transducer. MRI images of the resulting shear waves are processed to obtain an elastograph.
While this approach provides an initial step in the development of intracorporeal MRE, it suffers from the disadvantage of requiring that the ultrasonic transducer be mounted and powered at the distal end of the catheter (i.e. the end of the catheter that resides within the subject during imaging). This requirement adds significant complexity to the design of the system and necessitates powering the transducer by electrical leads integrated axially along the catheter. Such an arrangement is costly to produce, may be prone to an increased failure rate, and may also generate unwanted signal artifacts.
What is therefore required is a simple MRE system that does not require the incorporation of an active transduction device at the distal end of a catheter or probe.